iperf - A TCP, UDP, and SCTP network bandwidth measurement tool
Scapy - Scapy is a powerful interactive packet manipulation program.
netperf - Netperf is a benchmark that can be used to measure the performance of many different types of networking. It provides tests for both unidirectional throughput, and end-to-end latency. - HewlettPack...
Winsock Packet Editor - WPE Pro is a packet editor.
jsPerf - A performance playground for JavaScript developers. Easily create and share test cases and run cross-browser benchmarks to find out which code snippet is most efficient.
Nipper - Nipper - Neat Internet Protocol Packet EditoR - is a graphical packet generation tool.